1. Field of the Invention
The present invention relates to a radiation efficiency measuring apparatus and a radiation efficiency measuring method for measuring a radiation efficiency of an antenna for use in a cellular phone or the like.
2. Description of the Related Art
An apparatus for measuring a radiation efficiency or the like of an antenna used in a cellular phone or the like is known and generally discussed in, for example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2000-214201, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2000-338155, and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2-163668.
Japanese Unexamined Patent Application Publication No. 2000-214201 discloses a measuring apparatus. In order to evaluate antenna performance for a short period of time, the disclosed measuring apparatus measures the radiation efficiency of a cellular terminal antenna as a target antenna with the number of segments toward an elevation angle of the cellular terminal antenna reduced.
Japanese Unexamined Patent Application Publication No. 2000-338155 discloses a measuring system. The disclosed measuring system employs two measuring antennas, one for horizontally polarized wave and the other for vertically polarized wave. The system measures quickly the data of two polarized waves of a target antenna by switching between the two measuring antennas at a high speed.
Japanese Unexamined Patent Application Publication No. 2-163668 discloses an electromagnetic radiation measuring device. The disclosed electromagnetic radiation measuring device measures an electromagnetic radiation over the whole solid angle of a target object to be measured, such as a cellular phone, determines arrival probabilities of a horizontally polarized wave and a vertically polarized wave to the target object. The device then determines an effective radiation power using a weight function resulting from normalizing the arrival probability.
Japanese Unexamined Patent Application Publication No. 2000-214201 discloses that, if the number of segments to the elevation angle to the cellular terminal antenna is set to be 4, a measurement error of a radiation efficiency falls within about 3 dB at maximum. However, Japanese Unexamined Patent Application Publication No. 2000-214201 does not disclose a specific calculation method of the radiation efficiency. In addition, the measuring apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2000-214201 can result in a measurement error of about 3 dB, which is an undesirably low measurement accuracy.
The measuring system disclosed in Japanese Unexamined Patent Application Publication No. 2000-338155 measures a radiated electromagnetic field of the target antenna in the whole solid angle, and then determines the radiation efficiency by integrating the radiated electromagnetic field over the whole solid angle. For this reason, the measuring system disclosed in Japanese Unexamined Patent Application Publication No. 2000-338155 requires a long period of time for measurement. Moreover, a moving mechanism for moving the measuring antenna across the whole solid angle becomes bulky. According to Japanese Unexamined Patent Application Publication No. 2000-338155, the radiation efficiency is calculated by summing radiation powers over the whole solid angle. The radiation efficiency calculated is different from the originally defined radiation efficiency that is determined from calculating a ratio of a radiated power to an input power (radiated power/input power). Furthermore, the accuracy of the radiation efficiency calculated is not known.
The measuring device disclosed in Japanese Unexamined Patent Application Publication No. 2-163668 needs to measure the radiated electromagnetic field over the whole solid angle of the target object, such as the cellular phone, and thus requires a long measurement time as the measuring system disclosed in Japanese Unexamined Patent Application Publication No. 2000-338155.
In order to determine the effective radiation power, the measuring device disclosed in Japanese Unexamined Patent Application Publication No. 2-163668 also needs to determine the arrival probabilities of the horizontally polarized wave and the vertically polarized wave from data that is obtained by measuring the radiated electromagnetic field over the whole solid angle, and to calculate the radiation efficiency with the arrival probability weighted. The calculation method thus becomes complex, and the radiation efficiency cannot be calculated easily. Furthermore, since the radiated electromagnetic field is measured over the whole solid angle of the target object, the mechanism to control the target object to the elevation angle is required. The rotation mechanism thus becomes bulky and is difficult to be used in a compact anechoic chamber or the like.